Performance Enhancement of P-channel InGaAs Quantum-well FETs by - - PowerPoint PPT Presentation

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Performance Enhancement of P-channel InGaAs Quantum-well FETs by - - PowerPoint PPT Presentation

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Performance Enhancement of P-channel InGaAs Quantum-well FETs by Superposition of Process-induced Uniaxial Strain and Epitaxially-grown Biaxial Strain Ling Xia 1 , Vadim Tokranov 2 , Serge R. Oktyabrsky 2 , and Jess A. del Alamo 1 1

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SLIDE 1

Performance Enhancement of P-channel InGaAs Quantum-well FETs by Superposition of Process-induced Uniaxial Strain and Epitaxially-grown Biaxial Strain

Ling Xia1, Vadim Tokranov2, Serge R. Oktyabrsky2, and Jesús A. del Alamo1

1 Microsystems Technology Laboratories, MIT, USA; 2 College of Nanoscale Science and Engineering, SUNY-Albany, USA.

  • Dec. 06, 2011

Sponsors: Intel Corp. and FCRP-MSD Center. Fabrication: MTL at MIT.

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SLIDE 2

Outline

  • Motivation
  • Mechanical simulations
  • Device technology
  • Experimental results
  • Conclusions

2

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SLIDE 3
  • Interests in InGaAs CMOS – Fueled by excellent ve and µe
  • Key challenge for InGaAs CMOS
  • Bridging performance gap between n- and p-FET.
  • Our approach – Introduce strain to InGaAs p-FET
  • Uniaxial + biaxial compressive strain

Kuhn, IWJT, 2010 Logarithmic scale

Electron mobility Hole mobility

Motivation

del Alamo, Nature, 2011

3

slide-4
SLIDE 4
  • Sources for strain include:

– Epitaxial lattice mismatch  Biaxial strain – Fabrication process  Uniaxial strain

Why biaxial strain + uniaxial strain?

4

  • Enhancements of µh by biaxial and uniaxial strain add superlinearly
  • Similar effect found in Si simulations (Wang, TED, 2006)
  • 2.5
  • 2.0
  • 1.5
  • 1.0
  • 0.5

0.0 20 40 60 80 100 120

GaAs In0.24Ga0.76As

Max <110> piezoresistance coefficient (10

12 cm 2/dyn)

Epitaxial compressive biaxial strain (%)

Ge Gomez, EDL, 2010 Weber, IEDM, 2007 Xia, TED, 2011 Xia, ISCS, 2011

Experiments π<110> = ‐Δµ/(µ0σ <110>)

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SLIDE 5

InGaAs QW-FET with uniaxial + biaxial strain

  • Induced stress scalable with LG (next slide)

5

  • Biaxial strain in the

channel as grown

  • Carbon delta-doping
  • Self-aligned stressor

(compressively stressed)

  • Self-aligned metal

layer (Mo)

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SLIDE 6

Mechanical stress simulations

  • Parameters used in simulations: tSiN = 200 nm; SiN σint = -2 GPa

6

S D Cut line

LG = 2 µm

  • Desirable stress type can be obtained with the proposed

stressor structure

  • Compressive longitudinal stress  µh ↑
  • Tensile transverse stress  µh ↑

Lside = 100 nm

tSiN=200 nm x (µm)

  • 1

Longitudinal stress distribution

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SLIDE 7

0% 30% 60% 90% 120% 150% 180% 10 100 1000 10000

Δµh/µh0 LG (nm)

From From Total

Projected assuming Δµh/ µh = -π·σ

π// = - 1.2x10-10 cm2/dyn π = 0.7x10-10 cm2/dyn (Xia, ISCS, 2011)

σ σ//

  • 1,500
  • 1,200
  • 900
  • 600
  • 300

300 10 100 1,000 10,000

Stress (MPa) LG (nm)

Longitudinal Transverse Slow increase when LG >> tSiN Fast increase when LG ~ tSiN Slow down due to a fixed Lside

LG scalability of induced stress at middle of gate

  • LG ↓  Stress ↑ inside gate opening
  • Assume linear ∆µ with σ

 >160% µh enhancement for LG < 50 nm

7

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SLIDE 8

Device technology

8

Mesa isolation Ohmic metalization Molybdenum (Mo) deposition PECVD SiN stressor and SiO2 Anisotropic ECR RIE SiO2/SiN Anisotropic ECR RIE Mo Isotropic RIE Mo GaAs cap recess by wet etching Gate metalization

Key considerations:

  • Avoid Mo layer short to gate metal
  • Air gap as small as possible

Channel Barrier Buffer Al0.42Ga0.58As AlGaAs In0.24Ga0.76As

D S

SiN Mo SiO2

Cap : 2x1019 cm-3 Carbon doped GaAs

Channel Cap GaAs Buffer AlGaAs In0.24Ga0.76As

D S

SiN SiO2 Mo

Cap : 2x1019 cm-3 Carbon doped GaAs

SiO2

Barrier Al0.42Ga0.58As Channel Barrier Buffer Al0.42Ga0.58As AlGaAs In0.24Ga0.76As

D S

SiN Mo SiO2

Cap : 2x1019 cm-3 Carbon doped GaAs

Channel Cap Buffer AlGaAs In0.24Ga0.76As

D S

SiN SiO2 Mo

Cap : 2x1019 cm-3 Carbon doped GaAs

SiO2

Barrier Al0.42Ga0.58As

G

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SLIDE 9

Device cross-section

  • LG = 2 µm; channel along [-110]
  • Lside  400 nm

9 Artifact of imaging

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SLIDE 10

Experimental parameters for devices

  • Split experiments:
  • SiN with -2.1 GPa stress vs SiN with 0 Pa stress
  • SiN film stress obtained from wafer curvature

measurements

  • LG = 2 µm to 8 µm
  • Four channel orientations:

10

[-110] [110] S D G S D G [110] [-110] [010] [100]

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SLIDE 11

QW-FET electrical characteristics

  • Example: LG = 2 µm; channel along [-110]
  • Significant drive current increase
  • Transconductance increase at all gate overdrives

11

  • 2.0
  • 1.5
  • 1.0
  • 0.5

0.0 2 4 6 8 10

  • ID (mA/mm)

VDS (V)

  • 2.1 GPa SiN

0 GPa SiN VGS - VT = -0.75 V

  • 0.55 V
  • 0.35 V
  • 0.15 V
  • 0.5

0.5 10 20 30 40 VGS - VT (V) |gm| (mS/mm) 0 GPa SiN

  • 2 GPa SiN

VDS = -2 V

Intrinsic Extrinsic

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SLIDE 12

Subthreshold characteristics and VT

  • Similar IG as chemically etched samples No RIE damage
  • VT shift between high- and low- stress samples

– Likely due to different anisotropic RIE overetch

12

  • 0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

1E-5 1E-4 1E-3 0.01 0.1 1 10 With -2 GPa SiN

|I| (mA/mm)

LG = 2 m VDS = -2 V

ID

With 0 GPa SiN

VGS (V)

IG

Both S = 103 mV/dec

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SLIDE 13

LG dependence of gm

  • Increasing enhancement observed with decreasing LG
  • Consistent with stress simulations + π measurements
  • >160% enhancement expected with LG < 50 nm

13

2 4 6 8 5 10 15 20 25 30

|gm-sat| (mS/mm) LG (m)

VDS = -2 V VGS – VT = -0.5 V

+36%

Channel along [-110] Dots : data Lines: least-square fittings

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SLIDE 14
  • Observed anisotropic ∆gmi and ∆RSD

– gmi extracted using gmext, RS, RD and gD – RS, RD extracted using gate current injection method

  • <110> anisotropy consistent with measurements of

piezoresistance coefficients

– π[-110] : π [110] = 2.6 (Xia, ISCS, 2011)

Crystal direction dependence

14

gmi_sat

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SLIDE 15

kx (2/a) ky (2/a)

  • 0.1
  • 0.05

0.05 0.1

  • 0.1
  • 0.08
  • 0.06
  • 0.04
  • 0.02

0.02 0.04 0.06 0.08 0.1 No uniaxial

  • 500 MPa uniaxial

Theoretical discussions

  • Valence band change due to strain in InGaAs

– Used k.p methods (nextnano3) – Calculated subbands in In0.24Ga0.76As quantum well

15

In-plane iso-energy contours of hh1 with and without uniaxial strain [100] [010] [110]

  • Compressive strain parallel to

channel is desirable

  • 0.1
  • 0.05

0.05 0.1

  • 0.16
  • 0.14
  • 0.12
  • 0.1
  • 0.08
  • 0.06

Ev (eV)

k//[-110] k[110]

[-110] uniaxial No uniaxial

m*// ↓ m* ↑

hh1 lh1 hh2

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SLIDE 16

Effective mass model

  • Treat nonparabolic valence band using energy dependent

m* (De Michielis, TED, 2007)

From simulations:

  • ∆m* anisotropy induced by quantization change due to piezoelectric effect
  • ∆m* anisotropy consistent with gm measurements.

Gate

Pz(z)

Barrier Channel

[-110] Strain

  • 0.1
  • 0.05

0.05 0.1

  • 0.16
  • 0.14
  • 0.12
  • 0.1
  • 0.08
  • 0.06

Ev (eV)

k//<110> k<110>

[-110] uniaxial [110] uniaxial No uniaxial

m*// ↓ m* ↑

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SLIDE 17

Conclusions

  • Developed device architecture for InGaAs p-FETs that incorporates

uniaxial strain through self-aligned dielectric stressor

  • Key results:

– Biaxial strain + uniaxial strain  substantial enhancements in transport characteristics – Up to +36% ∆gm observed in LG = 2 µm device – Strong enhancement anisotropy due to piezoelectric effect

  • For further enhancement:

– Scale down LG and bring S/D closer – Project ∆gm >160% @ LG < 50 nm

  • Study useful to other p-type III-V materials (e.g. InGaSb, InSb)

17